Common thermoplastic resins, i.e., polycarbonates, polyesters, polyphenylene ethers, polyamides, polystyrenes, acrylics and the like have excellent combinations of properties making them suitable for use as molded articles. Such resins, in general, show good elongation, good tensile strength and good impact, among other properties. However, it is widely recognized that such resins are notch sensitive and subject to brittle failure upon impact due to poor resistance to crack propagation. This flaw in an otherwise excellent spectrum of physical properties greatly restricts the usefulness of articles molded from such resins.
Improving the notch sensitivity and preventing brittle failure upon impact of thermoplastic resins has been the subject matter of considerable research and development. Generally, the problem is addressed by the addition or admixture of additives to the thermoplastic resin which improve notch sensitivity without substantially affecting other properties. The most common type of such additives are rubber-like or elastomeric materials, such as ethylene-propylene copolymers (EPM), or ethylene-propylenepolyene terpolymers (EPDM), which form discrete particles dispersed throughout the thermoplastic resin. However, the desired level of improvement when achieved with the addition of such rubber-like or elastomeric materials results in deterioration of surface and internal appearance commonly achieved with the matrix resin by reason of the relative incompatibility between such rubber-like or elastomeric materials and many thermoplastic resins. This leads to an impact modified thermoplastic composition not suitable for many normal end uses of the matrix resin.
Attempts have been made to overcome this problem and increase the compatibility between the rubber-like or elastomeric materials and thermoplastic resins by modification of the rubber-like or elastomeric materials to provide such with sites that adhere to the thermoplastic resin and thus increase compatibility.
Cope, U.S. Pat. No. 3,435,093, discloses blends of polyethylene terephthalate and an ionic hydrocarbon copolymer of alpha-olefins of the formula R--CH.dbd.CH.sub.2 in which R is hydrogen (ethylene) or an alkyl radical of 1-3 carbon atoms (propylenepentene) with the copolymer modified with an alpha-beta ethylenically unsaturated carboxylic acid containing 3-5 carbon atoms. The Cope patent does not teach or suggest the components of the additive employed in the practice of the invention described and claimed herein, as will hereinafter appear.
The problem of improved toughness or ductility was faced directly in the Epstein U.S. Pat. Nos. 4,172,859 and 4,174,358. The Epstein patents are somewhat confusing in that they seek to cover the waterfront by listing an endless number of materials and combinations thereof for use as additives to improve the toughness and impact strength of polyamide, polyester and polycarbonate resins. In the Epstein patents, stress is placed on the particle size and tensile modulus of the copolymer additive. While Epstein contemplates the use of ethylene-propylene copolymers and ethylene-propylene-polyene terpolymers from amongst the large number of other varieties of materials and the use of alpha-beta ethylenically unsaturated carboxylic and dicarboxylic acids and anhydrides as modifying agents to provide sites which adhere to the matrix resin, the Epstein patents do not recognize the concepts of the invention described and claimed herein, as will hereinafter be pointed out.
In copending application Serial No. 690,613, of which this is an improvement, the invention described therein is based on the thought that an ethylene, C.sub.3 -C.sub.16 mono-olefin, polyene and preferably and ethylene, propylene, diene rubbery interpolymer would make a good impact modifier for such thermoplastic polyester, polyamide, polycarbonate resins if the two could be made compatible. The two are relatively incompatible because the rubber is a hydrocarbon while the thermoplastic or polyester is a much more polar substance. Thus, the objective of the invention described and claimed therein was addressed to the modification of the ethylene, mono-olefin, polyene interpolymer rubber greatly to improve its compatibility with polyester to provide an improved impact modifier for the thermoplastic polyester resin, while maintaining excellent internal and external appearance of the final molded material.
Briefly described, the features of the invention of the copending application are embodied in a composition comprising 60-90 percent by weight of the matrix thermoplastic resin such as in the form of a polyester blended with 10-40 percent by weight of an unsaturated rubber formed by copolymerization of ethylene, one or more mono-olefins and one or more polyenes in which the backbone unsaturated rubber component has been modified with an ester of an alpha-beta unsaturated acid having an epoxide functionality on the alkoxy portion, such as the ester derived from methacrylic acid and an epoxy alcohol and which attaches to the backbone rubber chiefly by way of a grafting reaction with little if any cross-linking reaction.
Copending Phadke application Ser. No. 800,332, filed Nov. 21, 1985, describes an improvement over the invention described and claimed in the aforementioned copending application Ser. No. 690,613, in that controlled cross-linking of the rubber backbone phase (EPDM) of the grafted modifier provides a significant improvement in the knitline strength of the final blend with the matrix (polyester, polycarbonate or polyamide) resin, when the cross-linking reaction is carried out after proper dispersion of the grafted rubber phase in the plastic matrix resin and when the cross-linking is concentrated between the rubber phase of the grafted rubber. For this purpose, use is made of a cross-linking agent in the form of a compound having functionalities capable of reaction with the grafted rubber, such as diacids or corresponding dianhydrides and/or diamines such as hexamethylene diamine (HDA), melamine, benzophenone tetracarboxylic dianhydride, adipic acid, maleic acid, maleic anhydride and the like.
While the desired improvements in impact strength, toughness and knitline strength have been achieved by blending a described thermoplastic matrix resin with modifiers of the types described in the aforementioned copending applications, the final products formed thereof, as by molding and the like, could be further improved from the standpoint of internal appearance and surface characteristics.